PRESSURE MEASURING DEVICESBY

ASHVANI SHUKLAC&I

RELIANCE

1

OVERVIEW

2

Pressure (P ) expresses the magnitude of normal force (F-N) per unit area (A-m2) applied on a surface(Crowe et al. 2005)

Units: Pa(= N/m2), psi(=lbf/in2), bar (=105 Pa=100 kPa), mbar (=100 Pa=1 hPa), atm (=101.3 kPa), mmHg (or Torr), inHg, etc. Note: For every Unit: hUnit=hectoUnit=100 Unit

Where Pabs : Absolute pressure Patm : Atmospheric pressure (standard is: 101.3 kPa =14.696 psi=760 mmHg=29.92 inHg) Pgage : Gage pressure

AFPor

AFP

gageatmabs PPP

PRESSURE MEASURING DEVICES

3

Bourdon Gage:

http://www.hydraulicspneumatics.com/FPE/images/sensors1_1.jpg

http://www.efunda.com/DesignStandards/sensors/bourdon_tubes/images/Bourdon_tube_A.gif

Applications: tire pressure, pressure at the top or along the walls of tanks or vessels

http://www.cpigauges.com/images/gauges/WeldGageStlCsBM400psi.jpg

Principles: change in curvature of the tube is proportional to difference of pressure inside from that outside the tube

PRESSURE MEASURING DEVICES

4

Strain Gage

Applications: Sensors for internal combustion engines, automotive, research etc.

Principles: ∆ P ∆ Resistance ∆ Voltage

PRESSURE MEASURING DEVICES

5

Quartz Gage

Applications: measurements with high accuracy, good repeatability, high resolution. e g. Quartz Clock

Principles: ∆ Pressure ∆ Charge ∆ Voltage

http://www.ransohoff.com/images/systems/transducerlgr.jpg

Piezoelectric transducers

http://upload.wikimedia.org/wikipedia/commons/c/c4/SchemaPiezo.gif

PRESSURE MEASURING DEVICES

6

Piezoresistive Gage

Applications: Very accurate for small pressure differentials e.g. Difference between indoor and outdoor pressure

Principles: ∆Pressure = ∆Charge = ∆Resistance = ∆Voltage

Digital Manometer

PRESSURE MEASURING DEVICES

7

U-tube Manometer

Principles: Hydrostatic Law

∆P=ρ g h

http://www.efunda.com/formulae/fluids/images/Manometer_A.gif

PRESSURE MEASURING DEVICES

8

U-tube Manometer

Air Water ManometerMercury Water Manometer

Applications: air pressure, pipe pressure, etc.

UT MANOMETER APPLET

9

Click here to connect to UT’s Interactive Fluids Applets website

Bellows Like a diaphragm, bellows are also used for pressure measurement,

and can be made of cascaded capsules. The basic way of manufacturing bellows is by fastening together many individual diaphragms. The bellows element, basically, is a one piece expansible, collapsible and axially flexible member. It has many convolutions or fold. It can be manufactured form a single piece of thin metal. For industrial purposes, the commonly used bellow elements are:

By turning from a solid stock of metal By soldering or welding stamped annular rings Rolling a tube By hydraulically forming a drawn tubing Working The action of bending and tension operates the elastic members. For

proper working, the tension should be least. The design ideas given for a diaphragm is applied to bowels as well. The manufacturer describes the bellows with two characters – maximum stroke and maximum allowable pressure. The force obtained can be increased by increasing the diameter. The stroke length can be increased by increasing the folds or convolutions.

10

For selecting a specific material for an elastic member like bellows, the parameters to be checked are:

Range of pressure Hysteresis Fatigue on dynamic operation Corrosion Fabrication ease Sensitivity to fluctuating pressures Out of these hysteresis and sensitivity to fluctuating

pressures are the most important ones. Hysteresis can be minimized by following a proper manufacturing technique. For instance, a diaphragm when machined from a solid stock shows less hysteresis compared to the one produced by stamping. The same technique could be adopted for bellows as well. In the latter case, the dynamic nature of the variable is likely to induce resonance quickly depending on the natural frequency of the system. The natural frequency is calculable from the dimensions of the system and the gauge.

11

For strong bellows, the carbon steel is selected as the main element. But the material gets easily corroded and is difficult to machine. For better hysteresis properties you can use trumpet bass, phosphor bronze, or silicon bronze. Better dynamic performance can be achieved by using beryllium copper. Stainless steel is corrosion resistive, but does not have good elastic properties. For easy fabrication soft materials are sought after.

All bellow elements are used with separate calibrating springs. The springs can be aligned in two ways – in compression or in expansion when in use. Both these types, with internal compression springs or external tension springs, are commercially known as receiver elements and are used universally in pneumatic control loops. The figures below show the compressed and expanded type. Spring opposed bellows are also shown below. The open side of a bellows element is usually rigidly held to the instrument casing and because of the rigid fixing, the effective or active length of the bellows element is smaller than its actual length. This device is used in cases where the control pressure range is between 0.2 to 1 kg/cm2.

12

13

Diaphragm

A diaphragm pressure transducer is used for low pressure measurement. They are commercially available in two types – metallic and non-metallic.

Metallic diaphragms are known to have good spring characteristics and non-metallic types have no elastic characteristics. Thus, non-metallic types are used rarely, and are usually opposed by a calibrated coil spring or any other elastic type gauge. The non-metallic types are also called slack diaphragm.

14

Working The diagram of a diaphragm pressure

gauge is shown below. When a force acts against a thin stretched diaphragm, it causes a deflection of the diaphragm with its centre deflecting the most.

15

Diaphragm Gauge Since the elastic limit has to be maintained,

the deflection of the diaphragm must be kept in a restricted manner. This can be done by cascading many diaphragm capsules as shown in the figure below. A main capsule is designed by joining two diaphragms at the periphery. A pressure inlet line is provided at the central position. When the pressure enters the capsule, the deflection will be the sum of deflections of all the individual capsules. As shown in figure (3), corrugated diaphragms are also used instead of the conventional ones.

16

17

Diaphragm Pressure Transducer Corrugated designs help in providing a linear deflection and also increase the

member strength. The total amount of deflection for a given pressure differential is known by the following factors:

Number and depth of corrugation Number of capsules Capsule diameter Shell thickness Material characteristics Materials used for the metal diaphragms are the same as those used for 

Bourdon Tube. Non-metallic or slack diaphragms are used for measuring very small pressures.

The commonly used materials for making the diaphragm are polythene, neoprene, animal membrane, silk, and synthetic materials. Due to their non-elastic characteristics, the device will have to be opposed with external springs for calibration and precise operation. The common range for pressure measurement varies between 50 Pa to 0.1 MPa.

The best example for a slack diaphragm is the draft gauge. They are used in boilers for indication of the boiler draft. The device can control both combustion and flue. With the draft, usually of pressure less than the atmosphere, connected, the power diaphragm moves to the left and its motion is transmitted through the sealing diaphragm, sealed link and pointer drive to the pointer.

The power diaphragm is balanced with the help of a calibrated leaf spring. The effective length of the spring and hence the range is determined by the range adjusting screw. By adjusting the zero adjustment screw, the right hand end of the power diaphragm support link as also the free end of the leaf spring, is adjusted for zero adjustment through the cradle. 18